A pocket-sizing apparatus for a device that inserts staple drivers into pockets aligned to form a slot in a surgical staple cartridge. A fin extends from a driver guide fixture for insertion into the slot. The fin increases the size of the slot, thereby aiding in inserting drivers into the pockets. Preferably a second fin extends from the driver guide fixture adjacent the fixture passage and aligned with the first fin. A gap is formed between the first and second fins, extending from a first edge to an opposite edge of the fixture passage. The driver is driven through the gap. The fins are substantially equal to the thickness of the driver. The fins have tapered leading edges to aid cartridge insertion and removal, and a lip extends from the driver guide fixture for preventing the fins from widening the slot too much and for narrowing an overly wide slot.